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Role of Angular Momentum in Riding a Bike

Author: Jerry Silver

The Idea

A bicycle is unstable when it is stationary. If you try to balance a bike that is not moving, it will fall. This experiment explores how it is possible to defy gravity and ride a bike.

What You Need

  • rope—about a meter (about a yard) in length
  • bicycle tire
  • dowel or section of broom handle to fit snuggly into the axle of the tire

Method

  1. Attach the rope to the axle of the bicycle tire. The tire should be able to turn freely around the attachment point. If there is no axle, insert a cylindrical piece of metal or a dowel for the tire to rotate around.
  2. Suspend the tire from the rope, so it tangles freely. Either hold the other end of the rope in your hand or attach it to something overhead.
  3. Holding the tire vertically by the axle, spin the tire.
  4. With the tire spinning, release the tire, so it is supported by the rope.
  5. Try this with the tire spinning rapidly and slowly, as shown in Figure 57-1.

Expected Results

The first thing to notice is the tire will remain in close to the vertical position. Also, it doesn't take much of a spin to keep the tire stable. You should also be able to observe that if left alone with the tire near the vertical position, the spinning tire rotates about the pivot point established by the rope. This is called precession.

Angular momentum. Riding a bike.

Why It Works

A full explanation of this simple situation can get complicated in a hurry. The basic idea is that a spinning tire has angular momentum. Gravity tries to rotate the tire from the vertical position that it is spinning in to the horizontal position that a non-rotating tire would be in. The force exerted by gravity produces a torque that is at right angles to both the force exerted by gravity and the direction of the angular momentum, which is along the line of the axle. This not only keeps the wheel from falling, but it also causes it to precess in a circle. This forms the basis for gyroscopic movement.

Other Things to Try

Like a bicycle, a toy gyroscope becomes stable only when it has sufficient angular momentum to counterbalance the pull of gravity. As an extension, study a gyroscope. Observe what happens when its turning axis is displaced from a stable position. What affects the rate of precession?

The Point

A bicycle tire is stable when it is rotating because the tire has angular momentum. The gravitational attraction of the Earth exerts a force that would pull the tire to a horizontal position were it not for the angular momentum of the spinning tire. The interaction of the torque caused by gravity and the angular momentum of the spinning tire results in the precession of the tire about the pivot point.

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